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Men, but not women, became hypersensitive to the mild pain from a heated plate when put in a room where they had previously been exposed to more intense pain from an inflated blood pressure cuff. (Shutterstock )

This is an excerpt from Second Opinion, a weekly roundup of eclectic and under-the-radar health and medical science news emailed to subscribers every Saturday morning.

Could the pain you feel in your body be all in your head? At least some of it might be — if you're a man (or a male mouse), a new study has found.

Male humans and male mice — but not females of either species — both became hypersensitive to pain when put in an environment where they previously had a painful experience, reports a new Canadian-led study published last week in the journal Current Biology.

"The sex difference was completely unexpected," said Loren Martin, the University of Toronto Mississauga assistant professor of psychology who led the study.

While there was no reason to believe males and females would respond differently, if they did, he would have expected females, not males, to develop pain hypersensitivity, since they're generally more sensitive to pain and more prone to chronic pain.

Martin originally ran an experiment on mice while he was a postdoctoral researcher in the lab of McGill University professor Jeffrey Mogil, who holds two research chairs related to pain.

They wanted to to see how the mice would react if brought back to a place where they had had a painful experience — a 30-minute tummy ache caused by dilute vinegar in their stomachs — and whether they could be conditioned to be hypersensitive to pain.

Pain and memory linked

The reason they were interested is because there is growing evidence that chronic pain is linked to biochemical "rewiring" in nerves similar to what happens with the formation of memories in the brain, and may itself be akin to or linked to memory, Martin said. If that's the case, chronic pain could potentially be treated by de-rewiring the nerves back to their normal state.

After seeing a surprising difference between males and females in an experiment on mice, the researchers decided to test if they would see a similar difference in men and women. (Shutterstock)

It wasn't a surprise that some of the mice became hypersensitive to less intense pain from a heated metal plate when put back in the container where they had had a stomach ache.

"The surprise was that this is only true in males," said Mogil.

At first, the researchers didn't believe the results. But after getting other researchers who didn't know about the sex difference to replicate the results several times, they decided to check if the same thing happened in humans.

Seventy-nine paid volunteers (mostly undergraduate students) were asked to put their hand on a hot metal plate and rate how painful it was. (Most rated it less than 50 on a scale of 0 to 100, with 100 being the most painful.)

What everyone's been doing for the last hundred years, which is only studying male mice or male rats, is obviously proving over and over again to be really dumb.- Jeffrey Mogil

Then, they were asked to wear an inflated blood pressure cuff for up to 20 minutes — something that hurt a lot more.

"It actually becomes quite painful," said Martin, who tried the blood pressure cuff on himself. The pain goes away immediately once the cuff is deflated.

The next day, to get the $50 they were promised (more than double the typical going rate for psych experiments), the volunteers had to come back, put their hand on the hot metal plate and rate their pain again.

Men put back in the room where they wore the blood pressure cuff rated the metal plate to be more painful than it had been the previous day, while everyone else rated it about the same.

Stress, testosterone play a role

The difference appears to be related to stress — men reported higher levels of stress from being in a room where they had had a painful experience.

Further tests in mice show that both memory and testosterone played a role in generating the sex-specific effect, but it's not clear exactly how.

The researchers are baffled about why the sex-specific difference exists in this case, although some people have suggested it might have to do with women being exposed to more pain than men, or that it's related to their ability to forget the pain of childbirth.

The study is just one of a growing number of studies showing that males and females experience and respond to pain differently, said Mogil.

He added that suggests "what everyone's been doing for the last hundred years, which is only studying male mice or male rats, is obviously proving over and over again to be really dumb."

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